1. Field of the Invention
The invention relates to aqueous dispersions of partially hydrophobic silicas, to a process for preparing them, to their use for stabilizing emulsions, and to their use as additives for rheology control of water-based coating materials, adhesives, and sealants.
2. Description of the Related Art
Aqueous dispersions of silicas find use in the chemomechanical planarizing of metal surfaces, in the semiconductor sector, for coating papers such as ink-jet papers, as rheological additives and/or antisedimentation agents in water-based paints, coatings, adhesives, and sealants, in the production of latex products such as gloves, in the production of gel batteries, and in the stabilizing of emulsifier-free Pickering emulsions.
The flow properties and the colloidal stability of aqueous silica dispersions are critically influenced by the pH. Thus, aqueous dispersions of silicas, particularly at pH levels in the neutral range, exhibit high viscosities and an inherent colloidal instability.
Aqueous silica dispersions are commonly stabilized electrostatically by altering the surface charge of the silica particles. Hence it is known from the specification of DE 40 06 392 that colloidally stable and low-viscosity dispersions of hydrophilic silicas can be obtained through establishing a pH in the basic range. A disadvantage here is that at pH levels in the region of the neutral point, i.e., at a pH of around 7, which is required for numerous applications, the dispersions exhibit an uncontrolled increase in viscosity or even gelling, as shown for example in D. Heath, T. F. Tadros, J. Colloid Interface Sci. 1983, 93, 320. A further lowering of the pH beyond the neutral point then leads to a further fall in the viscosity. This behavior on the part of aqueous dispersions of hydrophilic silicas has the disadvantage that even small changes in the pH, of the kind that may occur, for example, during the formulating of complex mixtures, can lead to uncontrollable fluctuations in the flow properties of the formulation.
It is known, furthermore, from the specifications EP 1 124 693 A1, EP 736489 and DE 102 38 463 A1 that aqueous silica dispersions can be stabilized using aluminum salts and, from U.S. Pat. No. 2,892,797, by aluminates. A disadvantage here is that in the region of the neutral point, i.e., at a pH of 7, these dispersions tend toward instability, which can lead to an uncontrolled increase in the viscosity or even to gelling. Furthermore, the addition of aluminum salts may have adverse consequences in certain applications, for example, such as in the coating of ink-jet papers and in the rheology control of water-based epoxy resins, for example.